1. Field of the Invention
The present invention is directed to a metal oxide varistor having heat protection, especially to a metal oxide varistor with an automatic switching-off feature that automatically opens a circuit in conditions of overheating due to sustained over-voltages.
2. Description of the Prior Arts
Metal oxide varistors are widely used in circuits as voltage protection elements and inrush-current-absorbing elements. Metal oxide varistors have the capability of clamping high transient voltages appearing on unconditioned power lines to a low level to protect electrical equipment or devices connected to the line. While the metal oxide varistors have a long life and have the ability to repeatedly clamp high transient voltage spikes to a safe level, the metal oxide varistors do eventually fail and ultimately, even if a catastrophic failure does not occur, the impedance of metal oxide varistors decreases to the point where they present a significant load, and eventually overheat and fail while emitting smoke and fumes.
Thus, the protection is generally provided to a metal oxide varistors by connecting the varistors across a power line in series with a current limiting fuse and/or a thermal fuse. If the temperature of the varistor increases beyond the rated temperature of the thermal-fuse, the thermal fuse will open, thereby removing the varistor from the circuit. Thermal-fuses used to protect electrical circuits from varistor failure are generally cylindrical in shape and are mounted on printed circuit boards on which the varistor is mounted with the fuse arranged adjacent and parallel to the varistor body. As long as the thermal protective fuse is physically close enough to the varistor, an increase in varistor temperature will increase the temperature of the thermal protective fuse, causing it to open. While these thermal protective fuses used to protect electrical circuits from varistor failure have been somewhat effective, varistors may overheat and fail if localized overheating occurs at a portion of the varistor body remote from the fuse. The varistor and surrounding areas may be destroyed before the temperature at the fuse increases sufficiently to cause the fuse open.
In a further known prior art device, a conventional metal oxide varistor and a thermal fuse are packaged together in encapsulation material to provide heat protection. However, when the temperature of the varistor increases beyond the thermal fuse's rated temperature and the thermal fuse opens, air in the fuse expands because of the heat, which may cause a spark to be generated as a result of a short circuit. The expanded air and the spark may explode the encapsulating material and the elements of the conventional metal oxide varistor and damage other elements in the circuit. Moreover, the explosion may generate a loud noise and startle anyone in the vicinity of the varistor.
To overcome the shortcomings, the present invention provides a metal oxide varistor with heat protection to mitigate or obviate the aforementioned problems.